The present invention relates to an amplifier for alternating signals in the form of an integrated circuit comprising on the same substrate at least one inverter having a first and a second field effect transistor with insulated gates and of complementary types of conductivity, said first and second transistors receiving on their gates said alternating signals respectively through two coupling capacitors.
Such an amplifier may be used, for example, to increase an alternating voltage of relatively small amplitude delivered by the quartz crystal oscillator of an electronic timepiece to a level suitable for controlling the divider stages normally connected to the output of the oscillator.
FIG. 1 shows a quartz crystal oscillator, the output voltage of which is delivered to the following stages by an amplifier A2, for alternating signals like the one of the present invention. For application to the field of horology, the amplifier A2 comprises generally field effect transistors with insulated gates which permits integration on the same substrate as the other electronic circuits of the timepiece. The current consumption of the amplifier must be kept very low. Known amplifiers which can be integrated are formed of at least an amplifier stage comprising an inverter having two field effect transistors of complementary type of conductivity operating in push-pull. Such amplifiers have the disadvantages either to have a relatively high natural current consumption or to operate satisfactorily only in relation with a very precisely determined supply voltage, or to present a low input resistance and an inadequate output signal with the edges of the output pulses having slowly rising slopes. FIG. 2 shows the circuit diagram illustrating the principle of an inverter with two transistors of complementary type of conductivity which can be utilized in an amplifier according to the present invention in which the drains D and the gates G are respectively connected together. A resistive element (e.g. a transmission gate) is connected between the gates and the drains. This element provides the bias voltage for the inverter so that the voltage gain of the latter is maximum at the choosen operating point F, as indicated in FIG. 3. FIG. 4 shows the current-voltage characteristics I.sub.DS =f(U.sub.GS) of the two transistors of P and N types of conductivity, respectively. In the case of alternating input signals of small amplitudes both transistors are conducting all the time so that in the example of FIG. 4 the current never falls below 50 nA. This current flows directly through both transistors and it does not contribute to the charge or discharge of the capacitor C2 representative of the load impedance of the inverter. For an input signal having a frequency of 32 kHz corresponding to a period of about 30 .mu. S, a charge of at least 3 pC per period of the input signal passes through the transistors, while a charge of 0.75 pC per period of the input signal would be enough to charge and discharge an external capacitor C2 of 0.5 pF. This shows that the circuit has an exaggerated consumption of current which, for a given geometry of the transistors, depends on the difference between the supply voltage U.sub.B and the sum of the threshold voltages V.sub.TP +V.sub.TN of the transistors of P and N types respectively. In practice, this difference can vary greatly, producing strong variations in current consumption by the amplifier, making such a circuit improper for utilization in any application requiring low current consumption.
FIG. 5 shows the behaviour of an amplifier when the supply voltage U.sub.B, is equal to the sum of the threshold voltages. According to the simplified theory of the field effect transistors by which no current flows in the transistor when the bias voltage between gate and source is smaller than the threshold voltage, the direct current in the amplifier of FIG. 5 must be zero. The current consumption is minimum and it depends on the load capacity C2 only. Operation of the amplifier according to the conditions of FIG. 5 is possible if the integrated circuit is provided with a voltage stabilizer. Moreover, a level adapter circuit is also necessary for adapting the level of the logic signals between the part of the amplifier operating at low supply voltage and the part under the full supply voltage.
FIG. 6 shows the principle of biasing for another known circuit which may be utilized in an amplifier for alternating signals in which, in the absence of an alternating input signal, no direct current flows in the transistors. FIG. 7 shows the circuit diagram illustrating the principle of an inverter with two transistors of complementary types biased as indicated in FIG. 6. The transistors T1 and T2 are separately controlled by the alternating input signal delivered by the coupling capacitors C3 and C4 and they are biased through two resistive elements R1 and R2 respectively by the threshold voltages V.sub.TP and V.sub.TN.
The certificate of utility FR No. 2 259 482 describes a circuit like the one of FIG. 8 in which the inverter comprising the transistors T1 and T2 of complementary type is biased by the transistors T5 to T8 in accordance with the principle illustrated in FIG. 6. FIG. 9 shows the simplified biasing circuit of the N-type transistor T2 of FIG. 8. It is to be seen that the transistor T7 is fed by a current source Io which is comprised of transistor T8 in FIG. 8. FIG. 10 shows the I.sub.DS =f(U.sub.GS) characteristic of the biasing transistor T7 of FIGS. 8 and 9. For a gate voltage practically equal to the threshold voltage V.sub.TN the current I.sub.DS is an exponential function of the gate-source voltage. To the operating point determined by a channel current of 10 nA corresponds a gate-source voltage defined by the characteristic of FIG. 10. The dimensioning of transistor T7 and of the current source Io can be done so that, without any alternating input signal, the voltage U.sub.G2 is about equal to the threshold voltage V.sub.TN of transistor T2. If a rectangular input signal U.sub.E is applied to the circuit through the capacitor C6, FIG. 10 shows that this input signal produces a mean current of 24.2 nA in transistor T7. However, due to the fact that the current source is delivering a current of 10 nA only, the gate voltage of the transistor must decrease until the mean current becomes again 10 nA. Therefore, there exists an effect of level variation due to which the positive half periods of the input signal cannot drive the amplifier transistor T2 noticeably beyond its threshold voltage. The transistor T2 is therefore only weakly driven so that it can deliver a weak current only. By taking into consideration the requirements of a low input capacity, the available range into which the dimensioning of the transistors of the circuit is possible is limited because, in practice, only transistors of minimum dimensions may be utilized. Due to the weak output current, the edges of the pulses of the output signal are rising slowly so that a following stage, e.g. an inverter like the one of FIG. 1, connected to the output of the circuit of FIG. 8 has a strong current consumption. This is the first disadvantage of the circuit of FIG. 8. It reveals itself only when the circuit is coupled to other elements or circuits within the integrated circuit and it makes impossible to use the circuit for extreme requirements of low current consumption imposed on the circuit.
FIG. 10 shows further that strong current peaks flow in the transistor T7. These current peaks represent an ohmic load for the generator of the pulse signals which, in the case of a regulated oscillator with low output voltage, corresponds to critical additional current consumption. This is the second disadvantage of the circuit of FIG. 8.